Family Alopiidae:

Thresher Sharks — 3 species

upper lobe of caudal fin long and curving, about as long as the
shark's body (the family name means "fox", another creature
renowned for its resplendent tail)

head short, snout moderately long and conical

mouth small, teeth small to moderately large

gill slits short, the last two above the pectoral fin base

pectoral fins long and narrow

eyes large (those of Alopias superciliosus are proportionately the
largest of any non-avian vertebrate)

second dorsal and anal fins small, both with pivoting bases

precaudal pits present; tail stalk thick without lateral keels

circulatory system modified, allowing retention of metabolic heat

scattered distribution in coastal and oceanic waters; cool temperate
to tropical zones of Atlantic, Pacific, and Indian oceans.

Common Thresher Shark (Alopias
vulpinus)

The whip-tailed threshers are among the most easily recognizable of sharks.
Despite their lack of obvious mackerel shark characteristics (such as
equal-lobed tail, narrow tail stalk supported by strong lateral keels, large
gill slits and jaws), thresher sharks have traditionally been regarded as
closely allied with the great white and other mackerel sharks of the family
Lamnidae. However, the recent (1997) comparative mtDNA study of lamnoids by
Naylor et al. suggests that the threshers share a closer common ancestor with
the ragged-tooth sharks (Odontaspididae) and a
second group composed of the megamouth (Megachasmidae)
and crocodile sharks (Pseudocarchariidae).

Given their many shared specialized characters, it is rather surprising that
Naylor et al's molecular genetics data did not provide strong support for
monophyly (all derived from a common ancestor) of the thresher sharks. In some
cladograms representing the genetic data, the Bumpytail Ragged-Tooth Shark (Odontaspis
ferox) actually appeared within the Alopiidae; in others, the Megamouth
Shark (Megachasma pelagios) appeared within this group. These
unprecedented results suggest that all these sharks are genetically very
similar, having changed relatively little since they diverged from the common
ancestor of their meta-group. Compagno recently (1990) proposed a close affinity
between the Megamouth Shark and the threshers; this is intuitively appealing, as
the structure of the second dorsal fin, anal fin, and tail of both groups (Megachasmidae
and Alopiidae) is strikingly similar. The interpretation followed here (depicted
in the cladogram above) is that, 1) the odontaspidids
form a primitive sister taxon to the meta-group consisting of the alopiid,
megachasmid, and psudocarchariid sharks, and 2) the alopiids branch off next,
forming a primitive sister taxon to the megachasmid and pseudocarchariid sharks.

The alopiids apparently diverged fairly recently. The thresher shark lineage
first appears in the fossil record some 55 million years ago, represented by the
fossilized teeth of Alopias crochardi. At least two species of thresher
shark, the Common Thresher (Alopias vulpinus) and the Bigeye Thresher (Alopias
superciliosus), have portions of their circulatory system modified into a
counter-current heat exchanger, which allows them to retain metabolic heat. An
homologous (structurally similar) — though more extensively developed
— system
also occurs in the mackerel sharks. Thus, warm-bodiedness appears to have
evolved independently in two separate lineages of lamnoid (the families
Alopiidae and Lamnidae).

A recent (1995) study by Blaise J. Eitner using electrophoretic analysis of
serum proteins from all three recognized species of alopiid from several
populations suggests a new, as-yet unrecognized species of thresher shark from the
Pacific coast of Mexico. The potential new species is most similar to the common
thresher shark. Molecular genetics is revolutionizing our understanding of
marine biodiversity, frequently revealing that what we thought was a single,
widespread species may, in fact, be a series of "cryptic species" —
outwardly very similar, but genetically as distinct from one another as visibly
dissimilar species.